User equipment and base station participating in cell measurement procedures

ABSTRACT

The present disclosure relates to a radio base station serving a user equipment in a first radio cell of a mobile communication system. The radio base station comprises processing circuitry which determines that a neighbor radio base station does not provide a beamformed reference signal to the user equipment. The neighbor radio base station controls the transmission in its neighbor radio cell of an omni-directional reference signal and the beamformed reference signal. A transmitter transmits a reference signal request to the neighbor radio base station. The reference signal request requests the neighbor radio base station to provide the beamformed reference signal to the user equipment. A receiver receives from the neighbor radio base station a reference signal request response, including information on the transmission of the requested beamformed reference signal. The transmitter transmits a notification to the user equipment, comprising information on the requested beamformed reference signal.

BACKGROUND Technical Field The present disclosure is directed tomethods, devices and articles in communication systems, such as, 3GPPcommunication systems. Description of the Related Art

Currently, the 3rd Generation Partnership Project (3GPP) works at thenext release (Release 15) of technical specifications for the nextgeneration cellular technology, which is also called fifth generation(5G). At the 3GPP Technical Specification Group (TSG) Radio Accessnetwork (RAN) meeting #71 (Gothenburg, March 2016), the first 5G studyitem, “Study on New Radio Access Technology” involving RANI, RAN2, RAN3and RAN4 was approved and is expected to become the Release 15 work itemthat defines the first 5G standard. The aim of the study item is todevelop a “New Radio (NR)” access technology (RAT) which operates infrequency ranges up to 100 GHz and supports a broad range of use cases,as defined during the RAN requirements study (see, e.g., 3GPP TR 38.913“Study on Scenarios and Requirements for Next Generation AccessTechnologies”, current version 14.2.0 available at www.3gpp.org andincorporated herein its entirety by reference).

One objective is to provide a single technical framework addressing allusage scenarios, requirements and deployment scenarios defined in TR38.913, at least including enhanced mobile broadband (eMBB),ultra-reliable low-latency communications (URLLC), massive machine typecommunication (mMTC). For example, eMBB deployment scenarios may includeindoor hotspot, dense urban, rural, urban macro and high speed; URLLCdeployment scenarios may include industrial control systems, mobilehealth care (remote monitoring, diagnosis and treatment), real timecontrol of vehicles, wide area monitoring and control systems for smartgrids; mMTC may include the scenarios with large number of devices withnon-time critical data transfers such as smart wearables and sensornetworks. A second objective is to achieve forward compatibility.Backward compatibility to Long Term Evolution (LTE, LTE-A) cellularsystems is not required, which facilitates a completely new systemdesign and/or the introduction of novel features.

The fundamental physical layer signal waveform will be based on OFDM,with potential support of a non-orthogonal waveform and multiple access.For instance, additional functionality on top of OFDM such asDFT-S-OFDM, and/or variants of DFT-S-OFDM, and/or filtering/windowing isfurther considered. In LTE, CP-based OFDM and DFT-S-OFDM are used aswaveform for downlink and uplink transmission, respectively. One of thedesign targets in NR is to seek a common waveform as much as possiblefor downlink, uplink and sidelink.

Besides the waveform, some basic frame structure(s) and channel codingscheme(s) will be developed to achieve the above-mentioned objectives.The study shall also seek a common understanding on what is required interms of radio protocol structure and architecture to achieve theabove-mentioned objectives. Furthermore, the technical features whichare necessary to enable the new RAT to meet the above-mentionedobjectives shall be studied, including efficient multiplexing of trafficfor different services and use cases on the same contiguous block ofspectrum.

Since the standardization for the NR of 5^(th) Generation systems of3GPP is at the very beginning, there are several issues that remainunclear. For instance, there has been an ongoing discussion about how tosupport the downlink transmission of reference signals respectivelysynchronization signals for UEs. These reference/synchronization signalscan be used by the UEs for a variety of purposes such as cellsynchronization, performing measurements to determine the channelquality and/or in the context of RRM (Radio Resource Management)measurements for UE mobility (e.g., for UEs in RRC IDLE Mode and/or RRCCONNECTED Mode). It is important to establish and define processes thatallow the reference/synchronization signals to be fully used by the gNBsand UEs, so as to obtain the most benefits.

BRIEF SUMMARY

One non-limiting and exemplary embodiment facilitates providing animproved (cell) measurement procedure, where different entities (UE,gNBs) are participating.

In one general aspect, the techniques disclosed here feature a radiobase station serving a user equipment in a first radio cell of a mobilecommunication system. The radio base station comprises processingcircuitry which determines that a neighbor radio base station does notprovide a beamformed reference signal to the user equipment. Theneighbor radio base station controls the transmission in its neighborradio cell of an omni-directional reference signal and the beamformedreference signal. A transmitter of the radio base station transmits areference signal request to the neighbor radio base station. Thereference signal request requests the neighbor radio base station toprovide the beamformed reference signal to the user equipment. Areceiver of the radio base station receives from the neighbor radio basestation a reference signal request response, including information onthe transmission of the requested beamformed reference signal. Thetransmitter transmits a notification message to the user equipment,comprising information on the requested beamformed reference signal.

In one general aspect, the techniques disclosed here feature a radiobase station of a mobile communication system that controls transmissionof an omni-directional reference signal and a beamformed referencesignal in a first radio cell of the radio base station. The radio basestation comprises processing circuitry, which determines to transmitinformation on the beamformed reference signal to one or more neighborradio base stations. A transmitter of the radio base station transmits areference signal notification message to the one or more neighbor radiobase stations, comprising scheduling information of the beamformedreference signal to allow identifying the radio resources used by theradio base station to transmit the beamformed reference signal.

In one general aspect, the techniques disclosed here feature a userequipment in a mobile communication system. The user equipment comprisesprocessing circuitry, which determines that a neighbor radio basestation does not provide a beamformed reference signal to the userequipment. The neighbor radio base station controls the transmission inits neighbor radio cell of an omni-directional reference signal and thebeamformed reference signal. A transmitter of the user equipmenttransmits a reference signal request to a serving radio base stationserving the user equipment. The reference signal request requests theserving radio base station to request the neighbor radio base station toprovide the beamformed reference signal to the user equipment. Areceiver of the user equipment receives from the serving radio basestation a notification message, comprising information on the requestedbeamformed reference signal.

In one general aspect, the techniques disclosed here feature a methodfor operating a radio base station serving a user equipment in a firstradio cell of a mobile communication system. The method comprises thefollowing steps performed by the radio base station. It is determinedthat a neighbor radio base station does not provide a beamformedreference signal to the user equipment. The neighbor radio base stationcontrols the transmission in its neighbor radio cell of anomni-directional reference signal and the beamformed reference signal. Areference signal request is transmitted to the neighbor radio basestation, wherein the reference signal request requests the neighborradio base station to provide the beamformed reference signal to theuser equipment. From the neighbor radio base station a reference signalrequest response to the received, including information on thetransmission of the requested beamformed reference signal. Anotification message is transmitted to the user equipment, comprisinginformation on the requested beamformed reference signal.

In one general first aspect, the techniques disclosed here feature amethod for operating a user equipment in a mobile communication system.The method comprises the following steps performed by the userequipment. It is determined that a neighbor radio base station does notprovide a beamformed reference signal to the user equipment. Theneighbor radio base station controls the transmission in its neighborradio cell of an omni-directional reference signal and the beamformedreference signal. A reference signal request is transmitted to a servingradio base station serving the user equipment. The reference signalrequest requests the serving radio base station to request the neighborradio base station to provide the beamformed reference signal to theuser equipment. A notification message is received from the servingradio base station, comprising information on the requested beamformedreference signal.

It should be noted that general or specific embodiments may beimplemented as a system, a method, an integrated circuit, a computerprogram, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments will beapparent from the specification and Figures. The benefits and/oradvantages may be individually obtained by the various embodiments andfeatures of the specification and drawings, which need not all beprovided in order to obtain one or more of such benefits and/oradvantages.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the following exemplary embodiments are described in more detail withreference to the attached figures and drawings.

FIG. 1 shows an exemplary architecture for a 3GPP NR system,

FIG. 2 shows an exemplary user and control plane architecture for theLTE eNB, gNB, and UE,

FIG. 3 illustrates the user plane protocol stack for 5G NR,

FIG. 4 illustrates the control plane protocol stack for 5G NR,

FIG. 5 illustrates an exemplary signaling diagram for a X2 handoverprocedure of the LTE communication systems,

FIGS. 6 and 7 illustrates an overview of mobility-related RRM actionsfor respectively RRC IDLE and RRC CONNECTED,

FIG. 8 illustrates a scenario to explain the underlying problem where aneighboring gNB is not transmitting a beamformed NR connected referencesignal at all,

FIG. 9 illustrates a scenario to explain the underlying problem where aneighboring gNB is transmitting a beamformed NR connected referencesignal in a different direction than the direction in which the UE islocated,

FIG. 10 illustrates a scenario to explain the underlying problem where aUE has not the necessary scheduling information on a beamformed NRconnected reference signal transmitted by the neighboring gNB,

FIG. 11 illustrates a scenario to explain the underlying problem, wherea handover of the UE is adversely affected by a switched-off beam of theNR connected reference signal,

FIG. 12 illustrates the exemplary and simplified structure of a UE andan eNB,

FIG. 13 illustrates a signaling diagram for an improved inter-gNBcoordination procedure initiated by a serving gNB serving a UE,

FIG. 14 illustrates a signaling diagram for an improved inter-gNBcoordination procedure initiated by a UE,

FIG. 15 illustrates an exemplary MAC Control Element that can be use forthe UE-initiated reference signal request message,

FIG. 16 illustrates an exemplary PDCP Control PDU that can be use forthe UE-initiated reference signal request message, and

FIG. 17 illustrates a signaling diagram for an improved inter-gNBcoordination procedure initiated by a neighbor gNB.

DETAILED DESCRIPTION Basis of the Present Disclosure 5G NR SystemArchitecture and Protocol Stacks

As presented in the background section, 3GPP is working at the nextrelease for the 5^(th) generation cellular technology, simply called 5G,including the development of a new radio access technology (NR)operating in frequencies ranging up to 100 GHz. 3GPP has to identify anddevelop the technology components needed for successfully standardizingthe NR system timely satisfying both the urgent market needs and themore long-term requirements. In order to achieve this, evolutions of theradio interface as well as radio network architecture are considered inthe study item “New Radio Access Technology”. Results and agreements arecollected in the Technical Report TR 38.804 v14.0.0, incorporated hereinin its entirety by reference.

Among other things, there has been a provisional agreement on theoverall system architecture. The NG-RAN (Next Generation-Radio AccessNetwork) consists of gNBs, providing the NG-Radio access user plane(SDAP/PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminationstowards the UE. The gNBs are interconnected with each other by means ofthe Xn interface. The gNBs are also connected by means of the NextGeneration (NG) interface to the NGC (Next Generation Core), morespecifically to the AMF (Access and Mobility Management Function) bymeans of the N2 interface and to the UPF (User Plane Function) by meansof the N3 interface. The NG-RAN architecture is illustrated in FIG. 1,as taken from the TS 38.300 v.2.0, section 4 incorporated herein byreference.

Various different deployment scenarios are currently being discussed forbeing supported, as reflected, e.g., in 3GPP TR 38.801 v14.0.0incorporated herein by reference in its entirety. For instance, anon-centralized deployment scenario (section 5.2 of TR 38.801; acentralized deployment is illustrated in section 5.4) is presentedtherein, where base stations supporting the 5G NR can be deployed. FIG.2 illustrates an exemplary non-centralized deployment scenario and isbased on FIG. 5.2.-1 of TR 38.301, while additionally illustrating anLTE eNB as well as a user equipment (UE) that is connected to both a gNBand an LTE eNB (which is to be understood as an eNB according toprevious 3GPP standard releases such as for LTE and LTE-A). As mentionedbefore, the new eNB for NR 5G may be exemplarily called gNB.

An eLTE eNB, as exemplarily defined in TR 38.801, is the evolution of aneNB that supports connectivity to the EPC (Evolved Packet Core) and theNGC (Next Generation Core).

The user plane protocol stack for NR is illustrated in FIG. 3, ascurrently defined in TS 38.300 v0.2.0, section 4.4.1. The PDCP, RLC andMAC sublayers are terminated in the gNB on the network side.Additionally, a new access stratum (AS) sublayer (SDAP, Service DataAdaptation Protocol) is introduced above PDCP as described in sub-clause6.5 of S TS 38.300 v0.2.0. The control plane protocol stack for NR isillustrated in FIG. 4, as defined in TS 38.300, section 4.4.2. Anoverview of the Layer 2 functions is given in sub-clause 6, of TS38.300. The functions of the PDCP, RLC and MAC sublayers are listed insub-clauses 6.4, 6.3, and 6.2of TS 38.300. The functions of the RRClayer are listed in sub-clause 7 of TS 38.300. The mentioned sub-clausesof TS 38.300 v0.2.0 are incorporated herein by reference.

The new NR layers exemplarily assumed at present for the 5G systems maybe based on the user plane layer structure currently used in LTE(-A)communication systems. However, it should be noted that no finalagreements have been reached at present for all details of the NRlayers.

RRC States

In LTE, the RRC state machine consists of only two states, the RRC idlestate which is mainly characterized by high power savings, UE autonomousmobility and no established UE connectivity towards the core network,and the RRC connected state in which the UE can transmit user plane datawhile mobility is network-controlled to support lossless servicecontinuity.

The RRC in NR 5G as currently defined in section 5.5.2 of TR 38.804v14.0.0, incorporated herein by reference, supports the following threestates, RRC Idle, RRC Inactive, and RRC Connected, and allows thefollowing state transitions as defined in TR 38.804.

As apparent, a new RRC state, inactive, is defined for the new radiotechnology of 5G 3GPP, so as to provide benefits when supporting a widerrange of services such as the eMBB (enhanced Mobile Broadband), mMTC(massive Machine Type Communications) and URLLC (Ultra-Reliable andLow-Latency Communications) which have very different requirements interms of signalling, power saving, latency, etc. The new RRC inactivestate shall thus be designed to allow minimizing signaling, powerconsumption and resources costs in the radio access network and corenetwork while still allowing, e.g., to start data transfer with lowdelay.

LTE Handover Procedure

Mobility is a key procedure in LTE communication system. There are twotypes of handover procedures in LTE for UEs in active mode: theS1-handover and the X2-handover procedure. For intra-LTE mobility, thehandover via the X2 interface is normally used for the inter-eNodeBmobility. Thus, the X2 handover is triggered by default unless there isno X2 interface established or the source eNodeB is configured to useanother handover (e.g., the S1-handover) instead.

FIG. 5 gives a brief exemplary and simplified overview of the X2intra-LTE handover.

The X2 handover comprises a preparation phase (steps 4 to 6), anexecution phase (steps 7 to 9) and a completion phase (after step 9).The X2 intra-LTE handover is directly performed between two eNodeBs.Other entities of the core network (e.g., the MME, Mobility ManagementEntity) are informed only at the end of the handover procedure once thehandover is successful, in order to trigger a path switch to the neweNB. Step 2, termed Measurement Control, refers to the cell measurementprocedure and measurement reporting procedure performed between the UEand the serving eNodeB (here “Source LTE eNB”). As will become apparentlater, the present application provides improved procedures mainly withregard to the above step 2.

The status transfer message in step 8 provides the sequence number andthe hyper frame number which the target eNodeB should assign to thefirst packet with no sequence number yet assigned that it must deliver.

More information on mobility procedures in LTE can be obtained, e.g.,from 3GPP TS 36.331 v14.2.2, section 5.4 incorporated herein byreference, and from 3GPP 36.423 v14.2.0 section 8.2 incorporated hereinby reference.

LTE-(A)—Synchronization Signals, Reference Signals and RRM Measurements

A user equipment wishing to access an LTE cell must first undertake acell search procedure so as to determine time and frequency parametersthat are necessary to demodulate the downlink and to transmit uplinksignals with the correct timing. The cell search procedure in LTE beginswith a synchronization procedure which makes use of two speciallydesigned physical signals that are broadcast in each cell: the PrimarySynchronization Signal (PSS) and the Secondary Synchronization signal(SSS). These two signals not only enable time and frequencysynchronization, but also provide the UE with the physical layeridentity of the cell and a cyclic prefix length, additionally informingthe UE whether the cell uses Frequency Division Duplex (FDD) or TimeDivision Duplex (TDD).

The PSS and SSS are transmitted periodically, twice per 10 ms radioframe. In an FDD cell, the PSS is always located in the last OFDM(Orthogonal Frequency Division Multiplexing) symbol of the first and11^(th) slots of each radio frame, thus enabling the UE to acquire theslot boundary timing independently of the cyclic prefix length. The SSSis located in the symbol immediately preceding the PSS. In a TDD cell,the PSS is located in the third OFDM symbol of the 3r^(d) and 13^(th)slots, while the SSS is located 3 symbols earlier. The PSS and SSS aretransmitted in the central 6 resource blocks, enabling the frequencymapping of these synchronization signals to be invariant with respect tothe system bandwidth and thus allowing the UE to synchronize to thenetwork without any a-priori knowledge of the allocated bandwidth.

More detailed information on the LTE reference signals and LTEsynchronization signals (e.g., the PSS and SSS structure), can be found,e.g., in TS 36.211 version 14.2.0 sections 6.10 “Reference signals” and6.11 “Synchronization signals”, both of which are incorporated herein byreference.

Once synchronization between the eNodeB and UE has been achieved, LTE isa coherent communication system which uses equalization and detectionalgorithms exploiting the knowledge of the channel impulse response(CIR). Optimal reception by coherent detection typically requiresaccurate estimation of the propagation channel. For this purpose knownreference signals are inserted into the transmitted signal structure. Inorder to estimate the channel as accurately as possible, allcorrelations between channel coefficients in time, frequency and spaceshould be taken into account. Since reference signals are sent only onparticular OFDM resource elements (i.e., on particular OFDM symbols onparticular subcarriers), channel estimates for the resource elementswhich do not bear reference signals have to be computed viainterpolation.

In the LTE downlink, at least five different types of reference signalsare provided:

-   -   the Cell-Specific reference signals (often referred to as common        reference) signal;    -   the UE-specific reference signals (often known as DeModulation        Reference Signal, DM-RS);    -   the MBSFN-specific reference signals, which are used only for        multimedia broadcast single frequency network operation;    -   the Positioning reference signals, which from Release 9 onwards        may be embedded in certain positioning subframes for the purpose        of UE location measurements; and    -   Channel State Information (CSI) reference signals, which are        introduced in Release 10 specifically for the purpose of        estimating the downlink channel state and not for data        demodulation.

The UE-specific reference signals may be transmitted in addition to thecell-specific reference signals and are embedded only in the resourceblocks to which the PDSCH is mapped for those UEs. If UE-specificreference signals are transmitted, the UE is expected to use them toderive the channel estimate for demodulating the data in thecorresponding PDSCH resource blocks. A typical usage of the UE-specificreference signals is to enable beamforming of the data transmissions tospecific UEs. The eNodeB may use a correlated array of physical antennaelements to generate a narrow beam in the direction of a particular UE.The narrow beam experiences a different channel response between theeNodeB and UE, thus making use of the UE-specific reference signals toenable the UE to demodulate beamformed data coherently. In Release 9 theUE-specific reference signals have been newly designed in order toextend support to dual layer transmission, which may include thetransmission of two spatial layers to one UE.

In communication systems according to LTE, Radio Resource Management(RRM) encompasses a wide range of techniques and procedures, includingpower control, scheduling, cell search, cell reselection, handover,radio link or connection monitoring, and connection establishment andre-establishment. Cell Search within E-UTRAN (LTE) is one of the mostfundamental aspects of mobility and enables the UE to acquire thecarrier frequency, timing and Physical Cell Identity (PCI).

Further, RRM-related actions undertaken by the UE can be broadly dividedinto those relevant in the RRC-IDLE state and those relevant in theRRC-CONNECTED state. An overview of mobility related RRM actions for RRCIDLE and RRC CONNECTED is given in FIG. 6 and FIG. 7.

Mobility for RRC-IDLE UEs may involve the measurement and evaluation ofthe serving cell, the measurement of neighbor cells, the evaluation ofneighbor cells for cell reselection, the acquisition of the systeminformation of the target cell, and the cell reselection to the targetcell. Mobility for RRC-CONNECTED UEs distinguishes between differentscenarios, as depicted in FIG. 7.

The measurement of the serving cell and any neighboring cells is usuallyperformed in a regular manner by the UEs. This may also involve thetransmission of measurement reports from the UE to its service eNB.

The UE reports the measurement information in accordance with themeasurement configuration provided by E-UTRAN and applicable for a UE inRRC_Connected state. The measurement configuration can be provided tothe UE by means of dedicated signaling, e.g., using theRRCConnectionReconfiguration or RRCConnectionResume message. The UE canbe configured to report measurement information to the eNB so as tosupport the control of UE mobility. The following measurementconfiguration elements can be signaled via theRRCConnectionReconfiguration message.

Measurement objects: A measurement object define on what the UE shouldperform the measurements—such as a carrier frequency. The measurementobject may include a list of cells to be considered as well asassociated parameters (e.g., frequency or cell-specific offsets).

Reporting configurations: A reporting configuration consists of the(periodic or event-triggered) criteria which cause the UE to send ameasurement report, as well as the details of what information the UE isexpected to report (e.g., the quantities, such as Received Signal CodePower (RSCP) for UMTS or Reference Signal Received Power (RSRP) forLTE).

Measurement identities: These identify a measurement and define theapplicable measurement object and reporting configuration.

Quantity configurations: The quantity configuration defines thefiltering to be used on each measurement.

-   -   Measurement gaps: Measurement gaps define time periods when no        uplink or downlink transmissions will be scheduled so that the        UE may perform the measurements.

In LTE, several events A1-A5, B1, B2 are defined for event triggeredmeasurement reporting, such as when a serving cell becomes better thanan absolute threshold (A1) or becomes worse than an absolute threshold(A2), or a neighbor cell becomes better than an offset relative to theserving cell (A3). Events B1 and B2 are provided for inter-RAT (RadioAccess Technology) mobility, where event B1 is triggered when a neighborcell becomes better than an absolute threshold and event B2 is triggeredwhen a serving cell becomes worse than an absolute threshold andneighbor cell becomes better than another absolute threshold.

The E-UTRAN can influence the entry condition by setting the value ofsome configurable parameters used in these conditions, for example oneor more of the threshold, an offset, etc.

In addition to event-triggered reporting, the UE may be configured toperform periodic measurement reporting. In this case, some of theparameters may be configured as for the event triggered reporting,except that the UE starts reporting immediately rather than only afterthe occurrence of an event.

In a measurement report message, the UE only includes measurementresults related to a single measurement, in other words, measurementsare not combined for reporting purposes. If multiple cells triggered thereport, the UE includes the cells in order of decreasing value of thereporting quantity, i.e., the best cell is reported first.

More detailed information can be found in the technical specification3GPP TS 36.331, version 14.2.2 section 5.5 “Measurements” incorporatedherein by reference.

5G NR—Synchronization Signals, Reference Signals and RRM Measurements

Also for the 5G NR communication systems, synchronization signals andreference signals are expected to be necessary, but have to be designedto enable the UEs and gNBs to meet the diverse requirements imposed bythe new 5G NR technology.

It is expected that in NR beamforming will be widely used in higherfrequency, since at least the data channels must be beamformed toachieve the high data rates required for the new 5G radio technologysystems. Therefore, to enable a UE in RRC CONNECTED state to detect areference signal for RRM measurements while receiving beamformed datachannels, the reference signals may also be beamformed such that theirsignal strength and signal strength of the data channels are within theUE receiver's dynamic range. Moreover, in higher-frequency deploymentsthat rely on beamforming, the SINR (signal to interference plus noiseratio) could significantly drop quite fast, e.g., due to beam blockage,shadowing, etc. Thus, a robust mobility procedure would requiresynchronizations and/or reference signals for measurements to beavailable at least more frequently for UEs in RRC CONNECTED mode thanfor UEs in RRC IDLE mode.

It has been discussed so far with regard to the 5G NR communicationsystems that for RRM measurements for (Layer 3) mobility always-onreference signals would be used, at least for UEs in RRC IDLE state,that should also provide a substantially omni-directional coverage.Omni-directionality of the reference signal could be achieved by usingan omni-directional transmission pattern, which expands to alldirections of the radio cell at the same time. Alternatively,omni-directionality of the reference signal could also be achieved byusing a beam transmission pattern which is substantially only covering aspecific direction (area) of the radio cell, but where the beam is“sweeped” over the whole radio cell so as thus achieveomni-directionality within a particular time period needed for thesweeping process to reach the initial direction.

On the other hand, for RRC CONNECTED state UEs, the possibility of usingone or more additional reference signals, possibly transmitted in a beamfashion, is currently being discussed. The additional reference signalfor 5G NR could be similar to the CSI-reference signals already definedfor LTE (see above discussion), and/or may include a still furtherreference signal which is designed separately therefrom. Usingadditional and/or different reference signals by UEs in RRC CONNECTED ina flexible manner allows meeting the above-noted requirements while atthe same time allowing to control and limit the system overhead andlatency. Specifically, performing RRM measurements based on theadditional reference signal is found to be more accurate and cantherefore reduce the handover latency. One reason can be that theadditional reference signal is emitted from the same beam as the datatransmission, and/or that the additional reference signal is transmittedover a wider spectrum.

The reference signals currently envisioned for RRC IDLE could be thesame or similar to the synchronization signals or cell-specificreference signals used in LTE, as briefly discussed above, i.e., the PSSand/or the SSS. For instance, the NR IDLE reference signals could betransmitted in an omnidirectional manner, in contrast to any additionalNR CONNECTED reference signals which could beamformed (like the datachannels). The NR IDLE reference signals could be transmitted moresparsely compared to the NR CONNECTED reference signals. Furthermore,the NR CONNECTED reference signals could be turned on/off and/or couldbe configurable.

In summary, it is exemplarily assumed in the following that for 5G NRcommunication systems omni-directional reference signals (may also becalled synchronization signals, in view of the possible implementationsimilar to the LTE PSS and SSS) are used for RRM measurements by UEs inthe RRC IDLE state (in the following termed NR IDLE reference signals,for ease of reference). In order to obtain further benefits, it isfurther exemplarily assumed in the following that in NR at least oneadditional—beamformed—reference signal (possible similar to the LTECSI-RS), is used for RRM measurements by UEs in the RRC CONNECTED state(in the following termed NR Connected reference signals, for ease ofreference).

Moreover, in order to obtain the most benefits and comply with thedesign requirements for reference signals, a flexible resourceallocation could be foreseen for the additional reference signal. Forexample, the additional reference can be switched off by a gNB; forinstance, when there is no UE in RRC CONNECTED state being served in thearea served by the beam. In addition, the resources used fortransmitting the additional reference signal need not be fixed but canchange dynamically. This is different, e.g., from the synchronizationsignals (PSS, SSS) known from LTE, where the UE knows the position ofthe PSS and SSS within the configured frequency carrier. In order forthe UEs to know where the addition NR reference signals are transmitted,it may be provided with some configuration information (e.g., timeand/or frequency indications) from the network.

It should be noted that the 3GPP standardization for the new radiotechnology of 5G is ongoing and the terminology of the layers andentities as assumed above could be changed in the normative phase,without affecting the functioning of the embodiments of the invention.

As explained in the above paragraphs, the 5G cellular system iscurrently discussing how to design the reference signals in the radiocells for the UE mobility purpose in RRC Connected state and RRC Idlestate. At least two different reference signals are to be used for theRRM measurement purpose in the radio cell, the NR Idle reference signaland the NR connected reference signal. The NR connected referencesignal, in contrast to the NR Idle reference signal, will likely not betransmitted omni-directionally and further will not be always on (can beswitched off, e.g., to save energy or reduce interference). This canhowever lead to the following problems.

The following scenario is assumed for explaining the problem, inconnected with FIG. 8-11. A UE is currently being served by a gNB(serving gNB) and is located at the edge of serving gNB's radio cell.The UE can receive the reference signals transmitted by the serving gNB,exemplarily the NR connected reference signal (illustrated as a beam)and the NR idle reference signal (not illustrated separately, covers theradio cell).

On the other hand, with reference to FIG. 8, the neighboring gNB is nottransmitting the NR connected reference signal; e.g., it was switchedoff before to save power in view of that no UE in RRC Connected statewas being served by the neighboring gNB for said beam (or correspondingdata beam). Consequently, the UE is not able to receive the NR connectedreference signal, and has to perform the measurements of the neighboringcell only based on the NR idle reference signal which the neighboringgNB is assumed to be transmitting always.

Another problem could be that one or more NR connected reference signalsare indeed being transmitted by the neighboring gNB. FIG. 9 exemplarilyillustrates one beam, representing one NR connected reference signal.However, neither of the NR connected reference signal beams is directedto the UE's location, such that the UE is again not able to receive theadditional reference signal to improve the measurement results performedfor the neighboring cell.

In another problematic scenario, there would be indeed an NR connectedreference signal transmitted by the neighboring gNB that would reach tothe UE, as illustrated in FIG. 10. However, the UE is again not able toreceive the additional reference signal since it does not know the radioresources used by the neighboring gNB to transmit this additionalreference signal beam. This scenario assumes that the time-frequencyradio resources used for transmitted the NR connected reference signalare chosen by the neighboring gNB in a dynamic or semi-dynamic manner.

In any case, there may be many situation where a UE cannot receive theadditional reference signal (NR connected reference cell) of aneighboring cell and can thus not take advantage thereof for itsmeasurements performed for the neighboring cell, e.g., as part of amobility (handover) procedure, as will be briefly explained inconnection with FIG. 11.

In particular, FIG. 11 illustrates two radio cells of two gNBs (gNB1 andgNB2), with respective cell coverage of the NR connected referencesignals (illustrated as beams C1B1, . . . C2B3) and of the NR idlereference signals (illustrated elliptically as being omni-directional).It is further assumed that beam C2B1 is switched off (dashed-line beam).The UE is located at time 1 in the overlapping area of the two radiocells, thus receiving the NR idle reference signal C1 of gNB1, the NRidle reference signal C2 of gNB2, as well as the NR connected referencesignal C1B3 of gNB1. Correspondingly, the UE can perform cellmeasurements on both radio cells, although the measurements for theradio cell of gNB2 will only be based on the NR idle reference signalC2, since the NR connected reference signal C2B1 is switched off.

Here, it should be noted that 3 GPP is considering a requirement for themobility that gNBs (making the handover decision) shall only comparemeasurement results being performed based on the same type of referencesignal, meaning that the serving gNB1 may not be allowed to compare themeasurement performed on the C2 with the measurement performed based onC1B3. Assuming that the measurement results based only on the NR idlereference signal are less accurate, the gNB1 may not be able to make acorrect handover decision.

For example, at t1 gNB1 may decide to not handover assuming that themeasurement results performed by the UE respectively on C1 and C2 aresubstantially the same and the measurements result performed for C1B3 ishigher than the non-existing measurement result for C2B1 (=0). Assumingthe UE moves further in the direction of gNB2 and leaves the coveragearea of gNB1, at time t2, the measurement result performed based on C2is better (e.g., better signal quality) than the measurement resultperformed based on C1. On the other hand, at t2 the measurement resultfor C1B3 will still be higher than the measurement result for C2B1.Thus, it is unclear how the gNB1 will make the handover decision. gNB1may decide to trigger the handover of the UE to the radio cell of gNB2,in which case the gNB2 will switch on C2B1 (due to the presence of theUE in RRC

Connected state) and will finally serve UE using the beam of C2B1 at astill later time t3. A delay in the handover is thus introduced, wherethe UE may also suffer from QoS degradations between t1 and t3.Conversely, if gNB1 decides to still not handover, the UE will have toeventually declare Radio Link Failure, as the signal strength from gNB1will diminish considerably.

The present disclosure thus shall present solutions facilitating toovercome one or more of the disadvantages and/or meet one or more of therequirements mentioned above.

Detailed Description of Present Disclosure

In the following, UEs, base stations, and procedures will be describedfor the new radio access technology envisioned for the 5G mobilecommunication systems. Different implementations and variants will beexplained as well. The following detailed disclosure was facilitated bythe discussions and findings as described in the previous section “Basisof the present disclosure” and may be based at least on part thereof.

In general, it should be however noted that only few things have beenactually agreed on with regard to the 5G cellular communication systemsuch that many assumptions have to be made in the following so as to beable to explain the principles underlying the present disclosure in aclear manner. These assumptions are however to be understood as merelyexamples that should not limit the scope of the disclosure. A skilledperson will be aware that the principles of the following disclosure andas laid out in the claims can be applied to different scenarios and inways that are not explicitly described herein.

Moreover, terms used in the following are closely related to LTE/LTE-Asystems or to terminology used in the current study items for 3GPP 5G,even though specific terminology to be used in the context of the newradio access technology for the next 3GPP 5G communication systems isnot fully decided yet. Consequently, a skilled person is aware that thedisclosure and its scope of protection should not be restricted toparticular terms exemplary used herein for lack of newer or finallyagreed terminology but should be more broadly understood in terms offunctions and concepts that underlie the functioning and principles ofthe present disclosure.

For instance, a mobile station or mobile node or user terminal or userequipment (UE) is a physical entity within a communication network. Onenode may have several functional entities. A functional entity refers toa software or hardware module that implements and/or offers apredetermined set of functions to other functional entities of a node orthe network. Nodes may have one or more interfaces that attach the nodeto a communication facility or medium over which nodes can communicate.Similarly, a network entity may have a logical interface attaching thefunctional entity to a communication facility or medium over which itmay communicate with other functional entities or correspondent nodes.

The term “base station” or “radio base station” here refers to aphysical entity within a communication network. The physical entityperforms some control tasks with respect to the communication device,including one or more of scheduling and configuration. It is noted thatthe base station functionality and the communication devicefunctionality may be also integrated within a single device. Forinstance, a mobile terminal may implement also functionality of a basestation for other terminals. The terminology used in LTE is eNB (oreNodeB), while the currently-used terminology for 5G NR is gNB.

The term “omni-directional reference signal” refers to a referencesignal having effectively an omni-directional coverage, which might alsobe achieved by using “sweeping” beams that cover different areas atdifferent times however thus cover all the area of the radio cell aftereach full sweep. Omni-directional is thus to be understood, in contrastto “beamformed”, in that a beam merely covers a limited (maybe narrow)non-circular area of a radio cell. Correspondingly, the term “beamformedreference signal” refers to a reference signal that is transmitted (by agNB) in a limited (maybe narrow) non-circular area of a radio cell, thusintentionally not covering the complete radio cell coverage.

-   -   FIG. 12 illustrates a general, simplified and exemplary block        diagram of a user equipment (also termed communication device)        and a scheduling device (here assumed to be located in the base        station, e.g., the LTE eNB or the gNB in 5G NR). The UE and        eNB/gNB are communicating with each other over a (wireless)        physical channel respectively using the transceiver.

The communication device may comprise a transceiver and processingcircuitry. The transceiver in turn may comprise a receiver and atransmitter. The processing circuitry may be one or more pieces ofhardware such as one or more processors or any LSIs. Between thetransceiver and the processing circuitry there is an input/output point(or node) over which the processing circuitry, when in operation, cancontrol the transceiver, i.e., control the receiver and/or thetransmitter and exchange reception/transmission data. The transceivermay include the RF (radio frequency) front including one or moreantennas, amplifiers, RF modulator/demodulator and the like. Theprocessing circuitry may implement control tasks such a controlling thetransceiver to transmit user data and control data provided by theprocessing circuitry and/or receive user data and control data which isfurther processed by the processing circuitry. The processing circuitrymay also be responsible for performing processes of determining,deciding, calculating, measuring, etc. The transmitter may beresponsible for performing the process of transmitting. The receiver maybe responsible for performing the process of receiving.

A simple and exemplary scenario is assumed in the following, based onpreviously-discussed FIG. 11, where a UE is currently connected to itsserving gNB1 and moving within the corresponding radio cell, eventuallyreaching overlapping coverage areas of other neighboring radio cells,here gNB2. The UE is configured to perform some cell measurements, e.g.,on the current radio cell of gNB1, but also—if applicable—on neighboringcells, such as the neighbor radio cell of gNB2. One exemplary optionwould be to re-use LTE procedures for configuring the cell measurementsto be performed by the UE, as briefly explained before. Correspondingly,gNB1 might configure the UE using a Measurement Control message (e.g.,as part of an RRCConnectionReconfiguration message), including differentelements such as Measurement objects, Reporting Configurations,Measurement Identities, Quantity Configurations, Measurement Gaps, etc.

For example, the UE can be configured to perform the cell measurementson the NR idle reference signal and—where available—on the NR connectedreference signal. In one exemplary implementation, differentmeasurements are to be performed for each reference signal, e.g., byconfiguring different measurement objects.

According to one exemplary implementation, in order to receive adynamically-allocated NR connected reference signal transmitted fromgNB1, the UE can be provided with suitable information on the NRconnected reference signal such that the UE is able to know where saidreference signal is transmitted and how to receive same. There may bevarious different implementations of a dynamically-allocated NRconnected reference signal. For instance, the NR connected referencesignals could be dynamically transmitted on different radio resources inthe frequency and time domain. On the other hand, an NR connectedreference signal may be transmitted at fixed time instances or atspecific frequencies known by the UE in advance. The periodicity andduration of the NR connected reference signal transmission may vary ormay be fixed.

The particular implementation chosen for the dynamically-allocated NRconnected reference signal determines how much information needs to beprovided to the UE to enable same to receive the NR connected referencesignal transmitted by the gNB1. Invariant parameters (e.g., time,frequency, periodicity, etc.) can be known to the UE from, e.g., aninitial configuration by the gNB1 when the UE attaches to the cell, orfrom the program code of the UE (e.g., fixed by 3GPP specifications) orfrom other suitable mechanism. Conversely, parameters that varydynamically may need to be provided to the UE in a timely and suitablemanner.

As apparent from the exemplary scenario of FIG. 11, it is assumed thatgNB1 transmits the NR idle reference signal in an omni-directionalmanner (C1), thereby covering substantially the whole serving radio cellarea. Further, three beamformed NR connected reference signals (C1B1,C1B2, C1B3) are transmitted by gNB1 as illustrated in FIG. 11.Consequently, the UE being located in the area of beam C1B3 can performcell measurements on the serving radio cell based on the NR idlereference signal C1 as well as on the NR connected reference signalC1B3.

Moreover, gNB2 transmits also its own NR idle reference signal in anomni-directional manner (C2), and beamformed NR connected referencesignals (C2B2, C2B3). The third NR connected reference signal (C2B1) iscurrently switched off and thus not transmitted by gNB2. The cellmeasurements performed by the UE for the neighbor radio cell of gNB2thus only use the NR idle reference signal transmitted by gNB2 since theother beams C2B2, C2B3 are not available at the location of the UE.

The UE is configured to prepare and transmit measurement reports to theserving gNB1. Measurement reports could be triggered, e.g., on aperiodical basis or based on certain preconfigured events. Some eventstriggering the measurements have already been presented in connectionwith the LTE measurement reporting procedure above. Some or all of theseLTE events A1-A5, B1, B2 could be reused also for the measurementreporting procedure for 5G NR. Alternatively or in addition, otherevents may be defined to trigger a measurement report, e.g., inconnection with the additional reference signal, NR connected referencesignal. For instance, separate events could be defined for the NRconnected reference signal and NR idle reference signal.

According to an improved implementation of the measurement reportingprocedure, the measurement reports prepared and provided by the UE mayalso provide information as regards to whether the enclosed measurementresults were calculated based on the NR idle reference signal and/or theNR connected reference signal. For instance, for each measurement resultapart from providing the actual metric value (e.g., the RSRP valueand/or the RSRQ value) and possibly the cell ID of the measured cell(e.g., the neighbor cell; may not include the cell ID for the servingcell) and possibly a corresponding measurement ID, the measurementreport may provide a flag to indicate the basis of the calculation,i.e., whether the result was calculated based on the NR idle referencesignal or on the NR connected reference signal. In one exemplaryimplementation, the measurement flag MF=0 could mean that the associatedmeasurement result was calculated based on only the NR idle referencesignal, whereas the measurement flag MF=1 would conversely mean that theassociated measurement result was calculated based on the NR connectedreference signal.

A different improvement of the measurement reporting procedure does notneed make use of a measurement flag and thus avoids the overhead causedby said flag. Instead, the serving gNB configures the measurements to beperformed by the UE such that measurements performed based on the NRidle reference signal can be distinguished from measurements performedbased on the NR connected reference signal. For instance, the servinggNB defines one measurement object with a particular measurement(object) ID for measuring neighbor cell(s) based on the NR idlereference signal, while defining another measurement object with anothermeasurement (object) ID for measuring neighbor cell(s) based on the NRconnected reference signal. This could be achieved, e.g., by using acorresponding flag in the measurement control message for each (or some)measurement object, transmitted to the UE, so as to distinguish betweenthe two measurement bases. Therefore, the UE performs the cellmeasurements in compliance with the instructed measurementconfiguration(s) and then accordingly prepares a measurement reportincluding the results of the measurements. The measurement report canalso include a measurement result ID per measurement result, such thatthe serving gNB1, when receiving the measurement report, identify theassociation between the received measurement result ID and theconfigured measurement object ID and can derive therefrom whether themeasurement result was obtained based on the NR idle reference signal orthe NR connected reference signal.

Another implementation of the improved measurement reporting procedureis that the serving gNB configures the reporting to be performed by theUE such that reporting performed based on the NR idle reference signalcan be distinguished from the reporting based on the NR connectedreference signal. For instance, the serving gNB defines one reportingconfiguration with a particular configuration ID for reporting themeasurement based on the NR idle reference signal, while defininganother reporting configuration with another configuration ID forreporting neighbor cell(s) based on the NR connected reference signal.This could be achieved, e.g., by using a corresponding flag in themeasurement control message for each (or some) reporting configuration,transmitted to the UE, so as to distinguish between the two measurementbases. Therefore, the UE performs the measurement report in compliancewith the instructed reporting configuration(s) and then accordinglyprepares a measurement report including the results of the measurements.The measurement report can also include a measurement result ID permeasurement result, such that the serving gNB1, when receiving themeasurement report, identify the association between the receivedmeasurement result ID and the configured reporting configuration ID andcan derive therefrom whether the measurement result was obtained basedon the NR idle reference signal or the NR connected reference signal.

Using this improved measurement reporting procedure allows the servinggNB to distinguish the measurement basis of the measurements performedby the UE, i.e., whether the UE performed measurements based on the NRidle reference signal or the NR connected reference signal. Thisinformation may be for instance used by gNB1 to be able to compare thereference signal measurements of the same type, since this is importantbecause making the handover decision based on the comparison on the sametype of reference signal is more accurate and hence can benefit the userequipment more.

In order to improve the cell measurement procedure, differentadvantageous implementations of an inter-gNB coordination (hereexemplary between serving gNB1 and gNB2) will be explained in thefollowing, particularly facilitating to overcome the problems identifiedbefore. The following implementations of the inter-gNB coordinationshall provide the opportunity for the UEs to receive the NR connectedreference signals of neighboring cells, provided of course theneighboring cells indeed decide to support the transmission of the NRconnected reference signals for the UE.

In general, the inter-gNB coordination can be initiated by the UE side,the serving gNB side and/or the neighbor gNB side, as will be separatelypresented in the following. First, inter-gNB coordination initiated bythe serving gNB and the UE side will be explained, as they share most ofthe processes, in contrast to the neighbor-gNB-initiated inter-gNBcoordination.

The serving gNB will at some point determine that the user equipment isnot being provided with the NR connected reference signal off theneighbor cell, which can be detrimental for the measurement accuracy andwhich can lead to one or more of the problems already discussed above.Moreover, the fact that the neighbor gNB is not transmitting the NRconnected reference signal in the area where the UE is located mightalso be an indication that the neighbor gNB has already a high load(thus trying to save energy where possible) and might not be interestedin or capable of providing (data) coverage for another UE.

Having identified this potential source of problems, the serving gNB maydecide to request the neighbor gNB to provide the missing NR connectedreference signal to the UE. Correspondingly, the serving gNB willtransmit a reference signal request to the neighbor gNB, requesting toprovide the NR connected reference signal to the UE. This requestmessage may for instance include a cell identification of the neighborcell of the neighbor gNB. In addition, this request message mayoptionally also include mobility information of the user equipment, soas to allow the neighbor gNB to identify the position of the UE. Thereare various implementations on what this mobility information (couldalso be termed location information) contains. In one implementation thelocation of the UE is presented by its GPS (Global Positioning System)coordinates, which can be obtained from the UE.

Another implementation for the UE location information makes use of theLTE positioning protocol (LPP), which allows the gNB1 to determine theposition of a UE. The gNB1 can thus also obtain the GPS coordinatesusing the LPP protocol, e.g., from an Evolved-Serving Mobile LocationCenter which is in connection with the UE. A detailed definition of theLPP can be found in the 3GPP TS 36.355 specification, latest versionbeing 14.1.0, section 5 “LPP Procedures”, incorporated herein byreference and 3GPP TS 36.455 v14.1.0, incorporated herein by reference.

Another implementation of the UE's location information in the referencesignal request message is based on the measurement results provided bythe UE. In particular, the serving gNB can estimate from themeasurements results where the UE is located. For instance, this can bedone by comparing the measurement results performed by the UE forseveral cells, thus determining relative RSRP information. Although lessaccurate than the GPS coordinates, such location information couldsuffice to allow the neighbor UE to decide whether to provide therequested NR connected reference signal to the UE.

In a still further implementation of the UE's location information inthe reference signal request message, the gNB may simply include themeasurement report (part of it, or completely) into the reference signalrequest message. Then, the neighbor gNB—if necessary or interested—mayhave to derive the location of the UE from the measurement results inthe same or similar manner as just explained in detail for gNB1 for theprevious implementation.

The following table shows an exemplary implementation of the referencesignal request message.

Element Name Presence Description The Cells to be The neighboring cellsto be negotiated Negotiated with with  > Cell ID Mandatory MobilityInformation Optional The mobility information of the UE can of the UEbe: measurement report from UE, or the relative RSRP informationretrieved from the measurement report, or location information from LPP

The neighbor gNB in turn, after receiving the reference signal requestmessage from the serving gNB of the UE, may decide whether or not toprovide the NR connected reference signal to the UE. This decision bythe neighbor gNB may take into account various different considerations.For instance, the neighbor gNB may be already serving too many UEs ormay already be limited in the power output, such that in either case itmay decide against providing the NR connected reference signal to the UE(and later would have to reject anyway the possible handover requestfrom the serving gNB). On the other hand, the neighbor gNB may also takeinto account the location information of the UE when determining whetheror not to provide the NR connected reference signal to the UE. Forinstance, it can be exemplarily assumed that the NR connected referencesignal is already being transmitted by the neighbor gNB2, although inanother direction than the direction in which the UE is located. In saidcase, instead of transmitting an additional beam for the NR connectedreference signal, it may be sufficient to merely change the direction ofthe already available beam of the NR connected reference signal so as toallow the NR connected reference signal to reach the UE's location. Insaid case, the neighbor gNB may decide to provide the NR connectedreference signal to the UE since no extra power has to be expended; thechange of direction of the beam may even be limited to a particularduration of time.

According to another scenario, the neighbor gNB might also take intoaccount the interference that would be caused in the radio cell bytransmitting one or more beams for the NR connected reference signal.

As just explained, the neighbor gNB will be able to make a decision asto whether or not it should provide the NR connected reference signal tothe UE. In any case however, the neighbor gNB may respond to thereceived reference signal request to the serving gNB1, by providingsuitable information on the transmission of the requested NR connectedreference signal. The content of this reference signal request responsemessage will depend on the outcome of the decision to provide or notprovide the NR connected reference signal to the UE.

In case the neighbor gNB2 decides against providing the NR connectedreference signal to the UE, the response message will have to merelyinclude a corresponding information element on the rejection of therequest. Optionally, the serving gNB may then inform the UE about thisrejection, such that the UE shall continue to perform the measurementsonly on the basis of the NR idle reference signal and may thus skiptrying to perform the measurements on the unavailable NR connectedreference signal. Furthermore, the UE might then know no longer initiatethe inter-gNB coordination, since it was already rejected. Optionally,the UE might even stop measuring the NR connected reference signal ofthe serving cell, in case no other neighboring cell are transmitted theNR connected reference signal, since no meaningful comparison of themeasurements can be made.

The response message may include the cell ID of the neighbor cell suchthat the serving gNB is able to determine to which radio cell (and towhich request) the response refers.

On the other hand, in case the neighbor gNB2 decides in favor ofproviding the NR connected reference signal to the UE, the referencesignal response message may include the confirmation of the requestand—if necessary or advantageous—may include further informationrelating to the requested NR connected reference signal.

For instance, the reference request response may include schedulinginformation for the NR connected reference signal, so as to be forwardedto the UE such that the UE is able to receive the NR connected referencesignal. The scheduling information may indicate the radio resources inthe frequency and/or time domain that will be used by the neighbor gNB2to transmit the requested NR connected reference signal.

On the other hand, all or part of the scheduling information may not benecessarily included in the reference request response message. Forinstance, the frequency(ies) and/or the timing of the radio resourcesmay be fixed in the 3GPP standard or may already be known to the UE orgNB1 from previous information exchanges, making the need for a furtherexchange of the scheduling information obsolete. In response to aprevious reference signal request message for the same or another UE,the gNB2 may have already provided the scheduling information of therequested NR connected reference signal to the gNB1. The presence of thescheduling information in the reference signal response message is thusoptional.

Moreover, according to further exemplary implementations, the gNB2, eventhough deciding to provide the NR connected reference signal asrequested to the UE, may further decide to limit the duration of timethe NR connected reference signal is provided to the UE. This may bedone in order to be able to again save energy and/or to reduceinterference, while at the same time still providing the UE with theopportunity to perform accurate measurements of the neighbor cell. Afterthe duration of time elapses, the neighbor gNB can either switch off thebeamformed NR connected reference signal, or change back its direction.In said case, gNB2 may or may not include corresponding durationinformation into the reference signal request response message, thisduration information then allowing the gNB1 and the UE (after beingforwarded to same) to learn during which time the enhanced measurementscan be performed, using the NR connected reference signal.

The gNB2 may in addition or alternatively decide to transmit the NRconnected reference signal to the UE with a specific periodicity,instead of transmitting same all the time. Again, this may be beneficialfor the gNB2, e.g., to save energy and/or to reduce interference. Insaid case, gNB2 may or may not include corresponding periodicityinformation in the reference signal request response message, whichwould then allow gNB1 and the UE (after being forwarded to same) tolearn at which periodicity the NR connected reference signal istransmitted by gNB2.

The following table illustrates a particular exemplary implementation ofthis reference signal request response message.

Element Name Presence Description The List of Cells Responding The cellsof the neighboring gNB in to the Request which the NR connectedreference signal has been turned on (or is on)  > Cell ID Mandatory  >List of TRPs The TRPs in which the NR connected reference signal hasbeen turned on (or is on)   >> TRP Index Number Mandatory   >>Scheduling Info Optional The scheduling information for the NR connectedreference signal in both the time and frequency domain   >> DurationOptional The duration during which the TRP will keep transmitting the NRconnected reference signal   >> Periodicity Optional The periodicity ofthe NR connected reference signal

The TRP mentioned above, refers to a Transmission Reception Point and isused in the technical field to circumscribe a beam (or beamformedtransmission). Consequently, the above exemplary response signalincludes a list of beams transmitted in the neighbor cell by gNB2(identified by the associated cell ID), respectively including a beamindex number as a mandatory information element, and includingscheduling information, duration information and periodicity informationas an optional information elements.

As explained above, the gNB2, when deciding to provide the NR connectedreference signal to the UE, may decide either to establish a new beam totransmit the NR connected reference signal to the UE, or to change thedirection and/or transmission power of another beam with which the NRconnected reference signal is already being transmitted so as to be ableto reach the UE.

The serving gNB1 will be informed accordingly using a reference signalrequest response message as discussed above. Subsequently, the servinggNB1 may inform the UE by transmitting a notification message to the UE,comprising information with regard to the requested NR connectedreference signal. The content of this notification message can varysubstantially based on the implementation and assumed scenario. Thetransmission of the notification message might not even be necessary forscenarios in which the UE, without further information, is able toreceive the requested NR connected reference signal, after same has beenmade available by the neighbor gNB (e.g., switched on or an alreadyavailable beam of the NR connected reference signal has been redirectedtowards the UE).

On the other hand, the notification message may include necessaryscheduling information (in the frequency and/or time domain), and/orduration information, and/or periodicity information, all of which maybe necessary for the UE to be able to properly receive and process therequested NR connected reference signal.

One exemplary implementation of this notification message transmittedfrom the serving gNB to the UE is the RRCConnectionReconfigurationmessage used by the serving base station to provide a new measurementconfiguration to the UE, possibly including necessary information on therequested NR connected reference signal. For instance, a new measurementobject can be defined for the new NR connected reference signal, or apreviously defined measurement object can be adapted accordingly. Thenew or adapted measurement object may then indicate how to conduct themeasurements involving the NR connected reference signal.

Alternatively, the notification message could also be implemented in theform of a MAC control element or a control signal of the PHY layer.

In any case, the UE shall be able to receive the requested NR connectedreference signal. Accordingly, the UE will perform the cell measurementson the neighbor cell using this NR connected reference signal, thusimproving the accuracy of the measurement results then provided to theserving radio base station in a measurement report, which may then beused by gNB2 for instance in order to make a decision on whether or notto handover the UE to the neighboring cell. The problems during thehandover procedure explained above in connection with FIG. 11 will thusbe mitigated in view of the availability of improved measurements on theNR connected reference signal.

This improved inter-gNB coordination according to one of the variousimplementations explained above allows mitigating the problems arisingfrom the use of the NR connected reference signal. The NR connectedreference signal has specific characteristics, such as its limitedcoverage area (in the form of a beam), it is not always on (since it canbe switched off), and it can be dynamically or semi dynamicallyscheduled (e.g., radio resources in frequency and or time may change).These specific characteristics of this additional reference signal forUEs in the RRC connected state provide various advantages as explainedabove, such as increased flexibility, energy saving possibilities,interference mitigation possibilities, etc. By using the improvedinter-gNB coordination, these advantages of using an NR connectedreference signal with such characteristics can be maintained, while atthe same time allowing UEs to take full advantage of the NR connectedreference signal to improve measurement accuracy.

In the above, it was briefly explained that the serving gNB will at somepoint determine that the user equipment is not being provided by theneighbor cell with the NR connected reference signal, which can bedetrimental for the measurement accuracy and which can lead to one ormore of the problems already discussed above. As will be explained inthe following, conceptually there are at least two different solutionson how the serving gNB determines that the user equipment is not beingprovided with the NR connected reference signal. According to onesolution, this determination is done by gNB1 based on the measurementreports received from the UE (serving-gNB-initiated inter-gNBcoordination). According to the second solution, the UE will firstdetermine that it is not receiving the NR connected reference signal andwill thus initiate the inter-gNB coordination itself by triggering thegNB1 as will be explained in the following (UE-initiated inter-gNBcoordination).

As regards the serving-gNB-initiated inter-eNodeB coordination, theserving gNB is receiving measurement reports from the UE as explainedabove, e.g., using the improved measurement report format. The servinggNB will then analyze the content of the measurement report,specifically the result of the measurements performed by the UE for theneighboring cell. As explained above, the improved measurement reportmay include information (such as the flag or measurement ID) from whichthe serving gNB can distinguish which measurements have been performedon the NR idle reference signal and which measurements have beenperformed on the NR connected reference signal. For instance, if theserving gNB determines that the UE performed measurements for theneighbor cell only based on the NR idle reference signal, but not basedon the NR connected reference signal, it may decide to initiate theinter-gNB coordination in order to request the neighboring cell to alsoprovide the NR connected reference signal to the UE.

More specific exemplary implementations define one or more particularevents which could trigger the serving gNB to initiate the inter-gNBcoordination as explained above. For instance, one event could be thatthe UE did not report a measurement of the NR connected reference signalof a specific neighbor cell for a long time. Another event could be thatthe UE did not report a measurement of the NR connected reference signalof a specific neighbor cell, and the link quality between the UE and theserving gNB becomes worse than a specific threshold. Another exemplaryevent could be that the UE did not report a measurement of the NRconnected reference signal of a specific neighbor cell, and the UEmobility (e.g., the moving speed) goes beyond a certain threshold.

As regards the UE-initiated inter-gNB coordination, the UE determinesthat it does not receive the NR connected reference signal of a neighborcell and thus decides to initiate the inter-gNB coordination in order torequest the neighbor gNB to provide the NR connected reference signal tothe UE. For instance, the UE receives the NR idle reference signal andthus knows that it is within coverage of a neighboring radio cell,however also notices that it does not receive a corresponding NRconnected reference signal for this neighboring radio cell. If the UEwants to improve the measurement accuracy based on the NR connectedreference signal, it may decide to transmit a corresponding UE-initiatedreference signal request to its serving gNB, which may indicate theradio cell or radio cells for which the UE would like to also receivethe NR connected reference signal. In turn, the serving gNB, uponreceiving the request from the UE, can decide whether or not to forwardthe request further to the neighbor gNB for which the request isintended. For instance, the serving gNB might already know that theneighbor gNB will not be able to serve another UE, or might know thatthe neighbor base station is actually an eNB not an gNB. In both cases,the serving gNB might decide to not agree with the UE's request.

It is exemplary assumed that the serving gNB agrees with the UE'srequest to request the neighbor gNB to provide the NR connectedreference signal, and thus that the serving gNB transmits a referencesignal request to the neighbor cell as explained in detail before.

To summarize the above, the inter-gNB coordination can either beinitiated by the UE or the serving gNB. Corresponding exemplary andsimplified implementations of such solutions are respectivelyillustrated in FIGS. 13 and 14 and will be explained as an overviewbelow.

FIG. 13, which illustrates the message exchange between the UE, gNB1 andgNB2, relates to the inter-gNB coordination initiated by the serving gNB(see box “request gNB2 for NR connected RS”). As illustrated in FIG. 13,it is exemplarily assumed that the UE is initially configured to providemeasurements (see arrow “Measurement Control”). In compliance with thescenario assumed for FIG. 11, it is assumed that the serving gNB1transmits the NR idle reference signal as well as the NR connectedreference signal, both of which are received by the UE (see dashedarrows). In contrast, the neighbor gNB2 transmits the NR idle referencesignal (see dashed arrow), but not an NR connected reference signal tothe UE. In accordance with the initially configured measurementconfigurations, the UE will eventually provide a measurement report tothe serving gNB1, from which the serving gNB1 can determine that the NRconnected reference signal of the neighbor gNB2 is not received by theUE (see above for details). The serving gNB1 may thus decide to requestthe neighbor gNB2 to provide the NR connected reference signal to theUE, by transmitting a corresponding request (see arrow “NR connected RSrequest”). The neighbor gNB2 can then decide whether or not to providethe NR connected reference signal to the UE. A corresponding responsemessage, in compliance with the decision taken by the gNB2, is thentransmitted by the gNB2 back to the gNB1. In FIG. 13 it is furtherassumed that the neighbor gNB2 indeed accepts to provide the NRconnected reference signal to the UE and accordingly transmits same tothe UE, as illustrated by the dashed arrow. The serving gNB1 can informthe UE on the result of the request, by e.g., transmitting a furtherMeasurement Control message (e.g., to provide scheduling information ofthe NR connected reference signal).

The UE can then proceed to perform the cell measurements also based onthe newly provided NR connected reference signal and provide themeasurement results to the serving gNB1 in a measurement report. Themeasurement results can then be used by the serving gNB1 to decidewhether to hand over the UE to the neighboring cell, in which case acorresponding handover request can be transmitted to the neighboringgNB2 in a suitable manner (e.g., as known from LTE). FIG. 13 ends withthe transmission of the handover request message, since further stepsare of less relevance to the ideas and implementations presented in thisapplication. The procedure may, e.g., continue in compliance with thetypical handover procedure in LTE as described with reference to FIG. 5.

FIG. 14 on the other hand relates to the inter-gNB coordinationinitiated by the UE and coincides in large parts with the implementationdiscussed above in connection with FIG. 13. As can be appreciated fromcomparing FIG. 13 and FIG. 14, the main difference is that the UE (notthe serving gNB1) at some point decides to request the gNB2 to providethe NR connected reference signal to the UE (see box “request gNB2 forNR connected RS”) and thus transmits the UE-initiated NR connectedreference signal request (see arrow “UE NR connected RS request”) to theserving gNB1. In turn, as exemplary assumed in FIG. 14, the serving gNB1decides to follow the UE's request and transmits the NR connectedreference signal request to the neighbor gNB2. The remaining steps inFIG. 14 can be the same or similar to the ones explained above inconnection with FIG. 13.

This UE-initiated reference signal request of FIG. 14 can be implementedin various ways, and in a basic exemplary solution may simply comprisethe cell ID of the neighbor cell (e.g., obtained from the NR idlereference signal) so as to identify the cell from which the UE wouldlike to receive the NR connected reference signal. Moreover, theUE-initiated reference signal request may also include various cell IDs,in case the UE would like to request the transmission of NR connectedreference signals from various neighboring cells.

In exemplary solutions, the UE may use a MAC Control Element as theUE-initiated reference signal request message. A specific LCID value(e.g., one of the non-reserved LCID values, e.g., 01100), known to boththe UE and gNB1, identifies the MAC Control Element as the UE-initiatedreference signal request. As already mentioned for the basic solution,the MAC control element may comprise information that allows the gNB1 toidentify the one or more cells for which the reference signal request ismeant. One solution for the cell identification could be to directlycarry the cell ID(s) within the MAC Control Element, which however mightincrease the size of the MAC Control Element substantially especiallyfor cases where the reference signal request refers to several radiocells.

Another solution for the cell identification could be to provide abitmap of cells, which in combination with the measurement report,allows the gNB1 to identify the radio cells for which the request ismeant. The size of the MAC Control Element can thus be reducedconsiderably. In particular, a bitmap of, e.g., 7 bits is provided inthe MAC Control Element, where each bit of the bitmap is associated witha particular measurement result in the measurement report. Themeasurement report in turn comprises for the measurement results for aneighbor cell also the cell ID of those neighbor cells, such that thebitmap identifies measurement results (and thus radio cells and theirIDs) for which the UE requests the transmission of the NR connectedreference signal. The UE has to take care that the cell bitmap in theMAC Control Element correctly points to those measurement results in themeasurement report that are associated with the radio cells for whichthe UE is requesting the transmission of the NR connected referencesignal.

An exemplary MAC Control Element is illustrated in FIG. 15, whichillustrates in the second octet a cell bitmap of 7 bits C1-C7, which canbe used to indicate (by setting the respective bit to, e.g., 1) to whichof the neighbor cells for which measurement results are included in themeasurement report the reference signal request should be transmitted.

In other exemplary solutions, the UE may use a PDCP Control PDU as theUE-initiated reference signal request message. A specific PDU type value(one of the non-reserved PDU type values, e.g., 100), known to both theUE and gNB1, identifies the PDCP control PDU as the UE-initiatedreference signal request message. As discussed in detail for the MACControl Element solution, the PDCP Control PDU may comprise informationthat allows the gNB1 to identify the one or more cells for which thereference signal request is meant. The two solutions discussed in detailabove for the cell identification in the MAC control element can beequally applied to the PDCP control PDU.

An exemplary PDCP Control PDU is illustrated in FIG. 16. As alreadyexplained in connection with FIG. 15, a cell bitmap of 7 bits C1-C7 isexemplarily assumed for the purpose of identifying the radio cells forwhich the UE is requesting the additional transmission of the NRconnected reference signal.

As mentioned above, instead of being initiated by either the serving gNBor the UE, the inter-gNB coordination can also be initiated via theneighbor gNB (here for instance gNB2). In said case, the neighbor gNB2will inform the serving gNB1 serving the UE (and possibly otherneighboring gNBs) about the transmission of the NR connected referencesignal in the radio cell of gNB2. This may include informing the othergNBs about the scheduling information, e.g., the radio resources in thefrequency domain and time domain, which will be used by gNB2 to transmitthe NR connected reference signal. This scheduling information may benecessary to enable UEs to receive the NR connected reference signalfrom the gNB2.

As explained before with regard to previous implementations, otherinformation may also be necessary for a UE to properly receive andprocess the NR connected reference signal. For instance, thetransmission of the reference signal might be restricted to a particularduration in time or might be performed with a specific periodicity.Correspondingly, duration information and/or periodicity information mayalso be provided to neighboring gNBs in addition to the schedulinginformation.

Even after providing the scheduling information (and possibly otherinformation) to other gNBs, the gNB2 may still decide whether or not tomaintain the one or more beams with which the NR connected referencesignal is transmitted. As already explained for previous solutions, gNB2might decide to switch off certain beams in order to save energy and/orreduce interference in its radio cell; particularly in case that no UEin RRC connected state is located within the coverage area of saidbeam(s). Alternatively, the gNB2 might also decide to change thedirection and/or the transmission power of certain beams, e.g., so as to(temporarily) provide the NR connected reference signal in a differentcoverage area. Conversely, gNB2 can at some point in time also decide toswitch on certain deactivated beams of the NR connected referencesignal.

In one exemplary implementation, so as to keep neighboring gNBs updatedon the status (e.g., switched off, switched on, changed direction, etc.)of the beams with which the NR connected reference signal is transmittedby gNB2, the gNB2 may decide to transmit further notification messagesto the neighboring gNBs. These subsequent notification messages could besimilar to the first notification message as explained above.

In another exemplary implementation, the subsequent notificationmessages can be simplified by merely identifying the one or more beamswhich status has changed (e.g., which have now been switched off or on,or have been redirected). To said end, the various beams with which theNR connected reference signal is transmitted are associated respectivelywith a beam index, uniquely identifying the beam transmitted by gNB2.The beam index can already be transmitted to the other gNBs togetherwith the scheduling information in the first notification message of theinter-gNB coordination. Then, subsequent notifications may merelyinclude the beam index or indexes of those beams which activation statusis changed, such that the receiving neighboring gNBs may derive whichbeams (with the corresponding scheduling information) are no longertransmitted (or are again available) from gNB2.

FIG. 17 illustrates an exemplary message exchange for an inter-gNBcoordination initiated by the neighboring gNB2. As illustrated, it isassumed that gNB2 decides to inform other neighbor gNBs about thetransmission of the NR connected reference signal. This may be done,e.g., upon starting the transmission of the NR connected referencesignal for the first time. Correspondingly, gNB2 transmits a referencesignal notification message to one or more neighboring gNBs (hereexemplarily to gNB1). The reference signal notification message maycorrespond to the one as explained above, and may thus include the cellID and optionally scheduling information about the NR connectedreference signal.

The neighboring gNBs, receiving the notification message from gNB2, canoptionally acknowledge the receipt of the notification message bytransmitting an acknowledgment message back to gNB2, as exemplarilyillustrated in FIG. 17.

The procedure illustrated in FIG. 17 may then exemplarily continue withtransmitting a measurement control message to the UE, e.g., so as toprovide the UE with information about the NR connected reference signalof gNB2. This measurement control message transmitted to the UE mayhowever not be necessary, e.g., in case the UE is not in the vicinity ofthe gNB2 or in case the UE does not have the capability of measuring theNR connected reference signal anyway.

FIG. 17 also illustrates the process of switching off the NR connectedreference signal (or at least one beam thereof), which can trigger thetransmission of another reference signal notification message, so as toinform one or more of the neighboring gNBs about the switched-offreference signal. As explained above, in one exemplary implementationthis subsequent notification may need to only include a beam index,identifying the beam that has been switched off. FIG. 17 assumesexemplarily that the UE is not informed about the switched off beam,e.g., because it is not relevant at that time.

The following table illustrates the content of an exemplary NR connectedreference signal notification message transmitted by gNB2 to otherneighboring gNBs.

Element Name Presence Description The List of Cells The cells in whichthe NR connected reference signal has been configured  > Cell IDMandatory  > List of TRPs The list of TRPs in which the NR connectedreference signal has been configured   >> TRP Index Number Mandatory  >> Scheduling Info Optional The scheduling information for the NRconnected reference signal in both the time and frequency domain   >>Periodicity Optional The periodicity of the additional RS

As apparent therefrom, it is assumed that the message as a mandatoryelement comprises the cell ID of gNB2, and further comprises a list ofbeams. For each beam, a beam index (TRP Index Number) is providedmandatorily and scheduling information and periodicity information areprovided as optional information elements.

As explained for previous solutions, the NR connected reference signalmight be scheduled dynamically, i.e., with changing radio resources.This would be however detrimental in the sense that the inter-gNBcoordination as initiated by the neighbor gNB would have to beadvantageously triggered each time the scheduling is dynamicallyadapted. Consequently, the inter-gNB coordination initiated by theneighbor gNB as explained above might be particularly advantageous forcases where the scheduling information for the NR connected referencesignal does not change often, such that the initially transmittedscheduling information remains valid for a long time.

Since the neighboring cells may be kept updated with the validscheduling information, the serving gNB1 is aware of the beams for theNR connected reference signal being transmitted by the gNB2 and can thusconfigure the measurements to be performed by the UE accordingly inadvance and in a fast manner. In said respect, the serving gNB1 cantransmit a measurement control message to the UE, e.g., when the UE isclose to the gNB2, and/or at an earlier point in time taking also intoaccount whether the UE's maximum number of measurement configurationshas not been reached.

As explained several times before, the UE will then perform the cellmeasurements in compliance with the measurement controlledconfigurations and will correspondingly transmit measurement reports toits serving gNB. The results of the measurement reports can then be usedfor instance by the serving gNB to decide on whether to hand over the UEto the neighboring cell. As exemplary illustrated in FIG. 17, theserving gNB1 can initiate the handover by transmitting the handoverrequest to the neighboring gNB2.

Several different solutions have been explained above for an improvedinter-gNB coordination, specifically being conceptually different as towhich entity initiates the inter-gNB coordination. Although thesedifferent solutions have been described separately from one another, itshould be noted that these can be also applied in parallel, i.e., incombination so as to obtain the most benefits of the respectivesolutions.

For instance, the inter-gNB coordination initiated by the neighbor gNBcan be applied to inform other gNBs in advance. In addition, exemplarilyassuming that the NR connected reference signal of gNB2 is not receivedby the UE, the UE-initiated inter-gNB coordination orserving-gNB-initiated inter-gNB coordination can be used to request thetransmission of an NR connected reference signal from gNB2. In responseto the request, gNB2 may then decide to switch on a beam to provide theNR connected reference signal to the UE or to appropriately change thedirection of another beam already providing the NR connected referencesignal. In view of that the necessary scheduling information is alreadyknown to gNB1 because of the previously-performed neighbor-gNB-initiatedinter-gNB coordination, the gNB2 does not need to again provide suchinformation in the response to the serving gNB. Furthermore, it mightnot be necessary anymore to properly configure the UE for themeasurements, in case the serving gNB had already provided correspondingmeasurement configuration information during the neighbor-gNB-initiatedinter-gNB coordination.

Similarly, the inter-gNB coordination might be initiated by the UE orthe serving gNB in parallel.

Further Aspects

According to a first aspect, a radio base station, serving a userequipment in a first radio cell of a mobile communication system isprovided. The radio base station comprises processing circuitry whichdetermines that a neighbor radio base station does not provide abeamformed reference signal to the user equipment. The neighbor radiobase station controls the transmission in its neighbor radio cell of anomni-directional reference signal and the beamformed reference signal. Atransmitter of the radio base station transmits a reference signalrequest to the neighbor radio base station. The reference signal requestrequests the neighbor radio base station to provide the beamformedreference signal to the user equipment. A receiver of the radio basestation receives from the neighbor radio base station a reference signalrequest response, including information on the transmission of therequested beamformed reference signal. The transmitter transmits anotification message to the user equipment, comprising information onthe requested beamformed reference signal.

According to a second aspect provided in addition to the first aspect,the reference signal request response informs that the neighbor radiobase station will not provide the requested beamformed reference signalto the user equipment. In one optional implementation, the notificationmessage transmitted to the user equipment informs on that the requestedbeamformed reference signal will not be provided to the user equipment.

According to a third aspect which is provided in addition to the firstor second aspect, the reference signal request response informs that theneighbor radio base station will provide the requested beamformedreference signal to the user equipment. In an optional implementation,the reference signal request response further includes schedulinginformation for the requested beamformed reference signal. In anoptional implementation, the scheduling information identifies radioresources in a frequency domain and/or a time domain. In an optionalimplementation, the notification message transmitted to the userequipment comprises scheduling information for the requested beamformedreference signal such that the user equipment can receive the requestedbeamformed reference signal.

According to a fourth aspect provided in addition to one of the first tothird aspects, the reference signal request includes information toderive the position of the user equipment. In an optionalimplementation, this information to derive the position includes globalpositioning system coordinates or relative measurement resultscalculated from the measurement report received from the user equipmentor the measurement report received from the user equipment. In anoptional implementation, the reference signal request further includesidentification information of the neighbor radio cell.

According to a fifth aspect provided in addition to one of the first tofourth aspects, the reference signal request response includes a list ofbeams with which the requested beamformed reference signal will betransmitted. In an optional implementation, for each beam in the listthe reference signal request response includes:

-   -   duration information about the duration of time the requested        beamformed reference signal will be transmitted by the neighbor        radio base station; and/or    -   periodicity information about the periodicity in time with which        the requested beamformed reference signal will be transmitted by        the neighbor radio base station.

According to a sixth aspect provided in addition to one of the first tofifth aspects, the receiver receives from the user equipment ameasurement report, including one or more results of measurementsperformed by the user equipment at least on the neighbor radio cell. Theprocessing circuitry determines that the neighbor radio base stationdoes not provide the beamformed reference signal to the user equipmentbased on the received measurement report and determines to transmit thereference signal request to the neighbor radio base station, in case theone or more measurement results on the neighbor radio cell compriseresults of measurements performed by the user equipment based on theomni-directional reference signal but do not comprise results ofmeasurements performed by the user equipment based on the requestedbeamformed reference signal. In an optional implementation, ameasurement result is associated with a flag in the measurement reportindicating whether the associated measurement result was calculatedbased on the omni-directional reference signal and/or the beamformedreference signal, and/or wherein a measurement result is associated witha measurement identification based on which the processing circuitry,when in operation, determines whether the associated measurement resultwas calculated based on the omni-directional reference signal and/or thebeamformed reference signal.

According to a seventh aspect provided in addition to one of the firstto sixth aspects, the receiver receives from the user equipment auser-equipment-initiated reference signal request requesting thebeamformed reference signal to be provided by the neighbor radio basestation. The processing circuitry determines that the neighbor radiobase station does not provide a beamformed reference signal to the userequipment based on the received user-equipment-initiated referencesignal request. The processing circuitry determines to transmit thereference signal request to the neighbor radio base station, based onthe received user-equipment-initiated reference signal request.

According to an eighth aspect provided in addition to the seventhaspect, the user-equipment-initiated reference signal request includesinformation to identify the neighbor radio cell and optionally includesinformation to identify at least a further radio cell for which acorresponding beamformed reference signal is requested. In an optionalimplementation, the user-equipment-initiated reference signal request isa Medium Access Control, MAC, Control Element, comprising apredetermined logical channel identifier identifying the MAC ControlElement to be the user-equipment-initiated reference signal request.Alternatively, the user-equipment-initiated reference signal request isa Packet Data Convergence Protocol, Control Packet Data Unit, a PDCPControl PDU, comprising a predetermined type identifier identifying thePDCP Control PDU to be the user-equipment-initiated reference signalrequest.

According to a ninth aspect provided in addition to one of the first tothe eighth aspects, the omni-directional reference signal is achieved bytransmitting same in all directions at substantially the same timeand/or by transmitting same in form of a beam which direction issuccessively changed in time so as to be thus transmitted in alldirections.

According to a tenth aspect provided in addition to one of the first toninth aspects, the radio base station controls transmission of a secondomni-directional reference signal in the first radio cell andtransmission of a second beamformed reference signal in the first radiocell. The receiver receives a second reference signal request fromanother radio base station requesting to provide the second beamformedreference signal to another user equipment. The processing circuitrydetermines whether or not to provide the second beamformed referencesignal to the other user equipment. In case of providing the secondbeamformed reference signal to the other user equipment, the radio basestation at least performs one of the following:

-   -   switching on the transmission of the second beamformed reference        signal, and/or changing the direction and/or the transmission        power of the second beamformed reference signal such that the        other user equipment is able to receive the second beamformed        reference signal; and    -   transmitting scheduling information to the other radio base        station for the requested second beamformed reference signal,        optionally wherein the scheduling information identifies radio        resources in a frequency domain and/or time domain used by the        radio base station to transmit the requested second beamformed        reference signal,    -   optionally, determining a duration of time during which the        second beamformed reference signal is to be provided to the        other user equipment.

In case of not providing the second beamformed reference signal to theother user equipment, the radio base station transmits a secondreference signal request response to the other radio base stationinforming that the radio base station will not provide the requestedsecond beamformed reference signal to the other user equipment.

According to an eleventh aspect provided in addition to one of the firstto tenth aspects, the processing circuitry determines whether tohandover the user equipment to the neighbor radio base station, based ona measurement report received from the user equipment comprising resultsof measurements performed by the user equipment for the neighbor radiocell. In the positive case, the transmitter transmits a handover requestto the neighbor radio base station.

According to a twelfth aspect, a radio base station of a mobilecommunication system is provided. The radio base station controlstransmission of an omni-directional reference signal and a beamformedreference signal in a first radio cell of the radio base station. Theradio base station comprises processing circuitry, which determines totransmit information on the beamformed reference signal to one or moreneighbor radio base stations. A transmitter of the radio base stationtransmits a reference signal notification message to the one or moreneighbor radio base stations, comprising scheduling information of thebeamformed reference signal to allow identifying the radio resourcesused by the radio base station to transmit the beamformed referencesignal.

According to a thirteenth aspect provided in addition to the twelfthaspect, the transmitter transmits the omni-directional reference signaland the beamformed reference signal in the first radio cell. In anoptional implementation, the processing circuitry determines whether ornot to switch off the transmission of the beamformed reference signal orwhether to change the direction of the beamformed reference signal.

According to a fourteenth aspect provided in addition to the twelfth orthirteenth aspects, the reference signal notification message comprisesidentification information of the first radio cell, and furthercomprises a list of beams with which the beamformed reference signalwill be transmitted. In an optional implementation, for each beam in thelist the reference signal notification message includes:

-   -   the scheduling information; and    -   duration information about the duration of time the beamformed        reference signal will be transmitted by the radio base station;        and/or    -   periodicity information about the periodicity in time with which        the beamformed reference signal will be transmitted by the radio        base station.

According to a fifteenth aspect provided in addition to one of thetwelfth to fourteenth aspects, the reference signal notification messageincludes an identification for the beamformed reference signal. Thetransmitter transmits another reference signal notification message tothe one or more neighbor radio base stations comprising theidentification of the beamformed reference signal to inform the one ormore neighbor radio base stations that the identified beamformedreference signal is now switched on or off.

According to a sixteenth aspect, a user equipment in a mobilecommunication system is provided. The user equipment comprisesprocessing circuitry, which determines that a neighbor radio basestation does not provide a beamformed reference signal to the userequipment. The neighbor radio base station controls the transmission inits neighbor radio cell of an omni-directional reference signal and thebeamformed reference signal. A transmitter of the user equipmenttransmits a reference signal request to a serving radio base stationserving the user equipment. The reference signal request requests theserving radio base station to request the neighbor radio base station toprovide the beamformed reference signal to the user equipment. Areceiver of the user equipment receives from the serving radio basestation a notification message, comprising information on the requestedbeamformed reference signal.

According to a seventeenth aspect provided in addition to the sixteenthaspect, the receiver receives the omni-directional reference signal andthe beamformed reference signal. The processing circuitry performsmeasurements on the omni-directional reference signal and the beamformedreference signal. The processing circuitry prepares a measurement reportincluding results of the performed measurements. In an optionalimplementation, a measurement result is associated with a flag in themeasurement report indicating whether the associated measurement resultwas calculated based on the omni-directional reference signal and/or thebeamformed reference signal, and/or wherein a measurement result isassociated with a measurement identification based on which the servingradio base station determines whether the associated measurement resultwas calculated based on the omni-directional reference signal and/or thebeamformed reference signal.

According to an eighteenth aspect provided in addition to the sixteenthor seventeenth aspect, the reference signal request transmitted by theuser equipment includes information to identify a neighbor radio cell ofthe neighbor radio base station and optionally includes information toidentify at least a further radio cell for which a correspondingbeamformed reference signal is requested. In an optional implementation,the reference signal request is a Medium Access Control, MAC, ControlElement, comprising a predetermined logical channel identifieridentifying the MAC Control Element to be the reference signal request.Alternatively, the reference signal request is a Packet Data ConvergenceProtocol, Control Packet Data Unit, a PDCP Control PDU, comprising apredetermined type identifier identifying the PDCP Control PDU to be thereference signal request.

According to a nineteenth aspect provided in addition to the eighteenthaspect, the information to identify the neighbor radio cell is a radiocell identification, or is a bitmap with two or more bits, each of thebits of the bitmap being associated to a result of a measurement in ameasurement report prepared by the user equipment for the neighbor cell,each result being associated with a radio cell identification of theneighbor radio cell.

According to a twentieth aspect, a method for operating a radio basestation serving a user equipment in a first radio cell of a mobilecommunication system is provided. The method comprises the followingsteps performed by the radio base station. It is determined that aneighbor radio base station does not provide a beamformed referencesignal to the user equipment. The neighbor radio base station controlsthe transmission in its neighbor radio cell of an omni-directionalreference signal and the beamformed reference signal. A reference signalrequest is transmitted to the neighbor radio base station, wherein thereference signal request requests the neighbor radio base station toprovide the beamformed reference signal to the user equipment. From theneighbor radio base station a reference signal request response to thereceived, including information on the transmission of the requestedbeamformed reference signal. A notification message is transmitted tothe user equipment, comprising information on the requested beamformedreference signal.

According to a twenty-first aspect, a method for operating a userequipment in a mobile communication system is provided. The methodcomprises the following steps performed by the user equipment. It isdetermined that a neighbor radio base station does not provide abeamformed reference signal to the user equipment. The neighbor radiobase station controls the transmission in its neighbor radio cell of anomni-directional reference signal and the beamformed reference signal. Areference signal request is transmitted to a serving radio base stationserving the user equipment. The reference signal request requests theserving radio base station to request the neighbor radio base station toprovide the beamformed reference signal to the user equipment. Anotification message is received from the serving radio base station,comprising information on the requested beamformed reference signal.

Hardware and Software Implementation of the Present Disclosure

The present disclosure can be realized by software, hardware, orsoftware in cooperation with hardware. Each functional block used in thedescription of each embodiment described above can be partly or entirelyrealized by an LSI such as an integrated circuit, and each processdescribed in the each embodiment may be controlled partly or entirely bythe same LSI or a combination of LSIs. The LSI may be individuallyformed as chips, or one chip may be formed so as to include a part orall of the functional blocks. The LSI may include a data input andoutput coupled thereto. The LSI here may be referred to as an IC(integrated circuit), a system LSI, a super LSI, or an ultra LSIdepending on a difference in the degree of integration. However, thetechnique of implementing an integrated circuit is not limited to theLSI and may be realized by using a dedicated circuit, a general-purposeprocessor, or a special-purpose processor. In addition, a FPGA (FieldProgrammable Gate Array) that can be programmed after the manufacture ofthe LSI or a reconfigurable processor in which the connections and thesettings of circuit cells disposed inside the LSI can be reconfiguredmay be used. The present disclosure can be realized as digitalprocessing or analogue processing. If future integrated circuittechnology replaces LSIs as a result of the advancement of semiconductortechnology or other derivative technology, the functional blocks couldbe integrated using the future integrated circuit technology.Biotechnology can also be applied.

Further, the various embodiments may also be implemented by means ofsoftware modules, which are executed by a processor or directly inhardware. Also a combination of software modules and a hardwareimplementation may be possible. The software modules may be stored onany kind of computer readable storage media, for example RAM, EPROM,EEPROM, flash memory, registers, hard disks, CD-ROM, DVD, etc. It shouldbe further noted that the individual features of the differentembodiments may individually or in arbitrary combination be subjectmatter to another embodiment.

It would be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present disclosure asshown in the specific embodiments. The present embodiments are,therefore, to be considered in all respects to be illustrative and notrestrictive.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary to employ concepts of thevarious patents, applications and publications to provide yet furtherembodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A radio base station serving a user equipment in a first radio cellof a mobile communication system, the radio base station comprising: areceiver, which in operation, receives a measurement report from theuser equipment, processing circuitry, which in operation, determinesbased on the measurement report that a neighbor radio base station doesnot provide a beamformed reference signal to the user equipment; and atransmitter, which in operation, transmits a reference signal request tothe neighbor radio base station, wherein the reference signal requestrequests the neighbor radio base station to provide the beamformedreference signal to the user equipment; wherein the receiver, when inoperation, receives from the neighbor radio base station a referencesignal request response; and the transmitter, when in operation,transmits to the user equipment a notification message comprisinginformation on the requested beamformed reference signal.
 2. The radiobase station according to claim 1, wherein a transmission in a neighborradio cell of an omni-directional reference signal and the beamformedreference signal is controlled by the neighbor radio base station. 3.The radio base station according to claim 1, wherein the measurementreport includes one or more results of measurements performed by theuser equipment on a neighbor radio cell.
 4. The radio base stationaccording to claim 1, wherein the reference signal request responseincludes information on a transmission of the requested beamformedreference signal.
 5. The radio base station according to claim 1,wherein the reference signal request response informs that the neighborradio base station will provide the requested beamformed referencesignal to the user equipment, wherein the reference signal requestresponse further includes scheduling information for the requestedbeamformed reference signal.
 6. The radio base station according toclaim 1, wherein the reference signal request response includes a listof beams with which the requested beamformed reference signal will betransmitted, wherein for each beam in the list the reference signalrequest response includes at least one of: duration information aboutthe duration of time the requested beamformed reference signal will betransmitted by the neighbor radio base station; and periodicityinformation about the periodicity in time with which the requestedbeamformed reference signal will be transmitted by the neighbor radiobase station.
 7. The radio base station according to claim 1, whereinthe receiver, when in operation, receives from the user equipment auser-equipment-initiated reference signal request requesting thebeamformed reference signal to be provided by the neighbor radio basestation, the processing circuitry, when in operation, determines thatthe neighbor radio base station does not provide a beamformed referencesignal to the user equipment based on the receiveduser-equipment-initiated reference signal request, and the processingcircuitry, when in operation, determines to transmit the referencesignal request to the neighbor radio base station, based on the receiveduser-equipment-initiated reference signal request.
 8. The radio basestation according to claim 7, wherein the user-equipment-initiatedreference signal request includes information to identify the neighborradio cell and includes information to identify at least a further radiocell for which a corresponding beamformed reference signal is requested.9. The radio base station according to claim 2, wherein theomni-directional reference signal is achieved by at least one of:transmitting a same in all directions at substantially the same time,and transmitting a same in a form of a beam whose direction issuccessively changed in time so as to be thus transmitted in alldirections.
 10. The radio base station according to claim 1, wherein theprocessing circuitry, when in operation, determines whether to handoverthe user equipment to the neighbor radio base station, based on themeasurement report comprising results of measurements performed by theuser equipment for a neighbor radio cell, wherein in a positive case,the transmitter transmits a handover request to the neighbor radio basestation.
 11. A communication method for a radio base station,comprising: receiving a measurement report from a user equipment,determining based on the measurement report that a neighbor radio basestation does not provide a beamformed reference signal to the userequipment; transmitting a reference signal request to the neighbor radiobase station, wherein the reference signal request requests the neighborradio base station to provide the beamformed reference signal to theuser equipment; receiving from the neighbor radio base station areference signal request response; and transmitting to the userequipment a notification message comprising information on the requestedbeamformed reference signal.
 12. The communication method to claim 11,wherein a transmission in a neighbor radio cell of an omni-directionalreference signal and the beamformed reference signal is controlled bythe neighbor radio base station.
 13. The communication method accordingto claim 11, wherein the measurement report includes one or more resultsof measurements performed by the user equipment on a neighbor radiocell.
 14. The communication method according to claim 11, wherein thereference signal request response includes information on a transmissionof the requested beamformed reference signal.
 15. The communicationmethod according to claim 11, wherein the reference signal requestresponse informs that the neighbor radio base station will provide therequested beamformed reference signal to the user equipment, wherein thereference signal request response further includes schedulinginformation for the requested beamformed reference signal.
 16. Thecommunication method according to claim 11, wherein the reference signalrequest response includes a list of beams with which the requestedbeamformed reference signal will be transmitted, wherein for each beamin the list the reference signal request response includes at least oneof: duration information about the duration of time the requestedbeamformed reference signal will be transmitted by the neighbor radiobase station; and periodicity information about the periodicity in timewith which the requested beamformed reference signal will be transmittedby the neighbor radio base station.
 17. The communication methodaccording to claim 11, comprising: receiving from the user equipment auser-equipment-initiated reference signal request requesting thebeamformed reference signal to be provided by the neighbor radio basestation, determining that the neighbor radio base station does notprovide a beamformed reference signal to the user equipment based on thereceived user-equipment-initiated reference signal request, anddetermining whether to transmit the reference signal request to theneighbor radio base station, based on the receiveduser-equipment-initiated reference signal request.
 18. The communicationmethod according to claim 17, wherein the user-equipment-initiatedreference signal request includes information to identify the neighborradio cell and includes information to identify at least a further radiocell for which a corresponding beamformed reference signal is requested.19. The communication method according to claim 12, wherein theomni-directional reference signal is achieved by at least one of:transmitting a same in all directions at substantially the same time,and transmitting a same in a form of a beam whose direction issuccessively changed in time so as to be thus transmitted in alldirections.
 20. The communication method according to claim 11,comprising: determining whether to handover the user equipment to theneighbor radio base station, based on the measurement report comprisingresults of measurements performed by the user equipment for a neighborradio cell, and in a positive case, transmitting a handover request tothe neighbor radio base station.